The present invention relates to the diagnostic imaging arts. It finds particular application in conjunction with magnetic resonance imaging gradient waveforms and will be described with particular reference thereto. It is to be appreciated that the present invention is also applicable to other types of waveform generation and is not limited to the aforementioned application.
In magnetic resonance imaging, a uniform main magnetic field is created through an examination region in which a subject to be examined is disposed. With open magnetic resonance systems, the main magnetic field is vertical, perpendicular to the subject. With classical bore systems, the main magnetic field is along the head to foot horizontal axis of a prone subject. A series of radio frequency (RF) pulses and magnetic field gradients are applied to the examination region to excite and manipulate magnetic resonances. Gradient magnetic fields are conventionally applied to encode spatial position and other information in the excited resonance.
The gradient fields are applied during an imaging sequence and typically treated mathematically as trapezoidal waveforms. Ideally, read gradient waveforms have a steep leading ramp which instantaneously transitions to a constant value for data sampling. After sampling, the gradient ramps steeply back to zero or to another value. As a practical matter, it is difficult to design a continuous waveform with sharp corners. Typically, these ideal waveforms are approximated by using sinusoidal waveforms with high frequency components to simulate corners. To generate such waveform corners, high voltages at high frequencies are needed. Equipment needed to generate such waveforms adds to system complexity and expense. Such waveform generation also increases the probability of gradient spiking, degrading the quality of the output image.
As stated before, it is difficult to construct a corner, i.e., a point whose derivative is undefined, out of a continuous waveform. Such waveforms have characteristic overshoots that oscillate to the desired value. Graphically, this manifests in a squiggle where a sharp corner should be. Typically, data is taken only during the portion of the waveform that is constant. This is done to avoid artifacts due to information being placed in incorrect regions of k-space. Such a data collection scheme is inefficient, as it is not utilizing the whole time frame of gradient activity.
The present invention provides a new and improved method and apparatus that overcomes the above referenced problems and others.
In accordance with one aspect of the present invention, a magnetic resonance imaging apparatus is given. A main magnet assembly produces a main magnetic field in an imaging region. An RF assembly excites and manipulates magnetic resonance. Gradient amplifiers drive gradient coils which spatially encode the magnetic resonance. A gradient optimizer optimizes gradient waveforms based on user input and hardware specifications. A reconstruction processor reconstructs received resonance.
In accordance with another aspect of the present invention, a method of magnetic resonance imaging is given. An imaging sequence is selected in which an RF pulse, a read gradient pulse, and at least one other gradient pulse are included. The read gradient is sampled and convolved with a band limited kernel matched to the gradient/amplifier system frequency response spectrum. The sequence is applied to generate magnetic resonance data, and the data is reconstructed into an image representation.
According to another aspect of the present invention, a method of diagnostic imaging is given. A subject is disposed in a main magnetic field of a magnetic resonance apparatus. An optimum gradient waveform is constructed and used in an imaging process where magnetic resonance is excited and received. The magnetic resonance is reconstructed and converted into an image representation.
According to another aspect of the present invention, a gradient optimizer is given. The gradient optimizer includes a model archive, a kernel generator, a spectrum memory, and a convolution circuit.
According to a more limited aspect of the present invention, the gradient optimizer can be utilized in new or pre-existing magnetic resonance systems.
One advantage of the present invention is the efficient reading of imaging data.
Another advantage is that it minimizes gradient spiking.
Another advantage is that it is backwardly compatible to existing systems.
Another advantage is improved gradient waveform fidelity for arbitrary types of k-space trajectories.
Another advantage is that it requires less computation for the design of the pulsed waveform (equivalently polygonal k-space trajectory).
Another advantage is the elimination of analog low-pass filters, reducing delay and increasing performance.
Another advantage is the elimination of numerical errors arising from interpolation, integration, and/or differentiation of sampled waveforms.
Yet another advantage is the efficiency of storing and transmitting the compact pulsed waveform representation.
Still further benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments.